The official KeePassX PPA does not update at the moment, but a third-party PPA has built the KeePassX 2.0 for Ubuntu 16.04 LTS, Ubuntu 15.04, and Ubuntu 14.04 LTS.

1. Add PPA

Open terminal from Unity Dash, App Launcher, or via Ctrl+Alt+T shortcut key. When it opens, paste below command to add the PPA:

sudo add-apt-repository ppa:eugenesan/ppa

Update: The 14.04 package so far is broken in the eugenesan/ppa. Use this PPA instead with yubikey support:

sudo add-apt-repository ppa:hda-me/keepassx2-yubico

Type in your password when it asks (no visual feedback just type in mind) and hit Enter to continue.

2. After that, upgrade KeePassX by running Software Updater. Or run below commands one by one in terminal:

sudo apt-get update
sudo apt-get install keepassx

For those who don’t want to add PPA, grab the .deb installer directly from HERE.

3. (Optional) As a third-party PPA, it also contains many other applications. You may remove the PPA after installation by either using Software & Updates -> Other Software tab, or running below command in terminal:

I'm a freelance blogger who started using Ubuntu 5+ years ago and wishes to share my experiences and some useful tips with Ubuntu beginners and lovers. Please notify me if you find any typo/grammar/language mistakes. English is not my native language. Contact me on Google Plus or email to ubuntuhandbook1@gmail.com

This ppa breaks trusty! do NOT install it if you are on trusty.
It replaces packages (eg metacity-common) with weird versions, thats incompatible with most of the system, that totally breaks mono and its apps like monodevelop.
ppa-purge cant handle conflicts, you’ll need to manually reinstall broken packages when you’ll remove this ppa, and there’s a plenty of them.